It can happen in an instant — your child takes a hard hit to the mouth while playing football, basketball or some other contact sport. Suddenly, he or she faces the severest of dental injuries: a knocked out tooth.

There's both good and bad news about this situation. First, the good news: the knocked out tooth can be reinserted into its socket and take root again. The bad news, though, is that the tooth has only the slimmest of chances for long-term survival — and those chances diminish drastically if the reinsertion doesn't take place within the first five minutes of the injury.

Outside of the five-minute window, it's almost inevitable that the tooth root won't reattach properly with the tiny fibers of the periodontal ligament, the sling-like tissue that normally holds the tooth in place to the jawbone. Instead, the root may fuse directly with the bone rather than via the ligament, forming what is called ankylosis. This will ultimately cause the root to melt away, a process known as resorption, and result in loss of the tooth.

Of course, the resorption process will vary with each individual — for some, tooth loss may occur in just a few years, while for others the process could linger for decades. The best estimate would be four to seven years, but only if the tooth receives a root canal treatment to remove any dead tissue from the tooth pulp and seal it from possible infection. Over time the tooth may darken significantly and require whitening treatment. Because the tooth may be fused directly to the jawbone it can't grow normally as its neighbor teeth will and thus may appear uneven in the smile line. From a cosmetic point of view, it may be best at that time to remove the tooth and replace it with an implant or other cosmetic solution.

In many ways the longevity of the tooth post-injury really depends on time — the time it takes to reinsert the knocked out tooth into its socket. The quicker you take action, the better the chances the tooth will survive.